This invention relates to a printing apparatus for printing on a substrate and to a method of printing. More particularly the invention relates to a printing apparatus which utilises a printing ribbon which includes a web carrying marking medium, a print head in use, removing marking medium from selected areas of the web to transfer the marking medium to the substrate to form an image, such as a picture or text.
More particularly but not exclusively the invention relates to a so called thermal printing apparatus in which the print head includes a plurality of thermal heating elements which are selectively energisable by a controller during printing to soften and remove pixels of marking medium from the ribbon and to transfer such pixels to the substrate. However the invention may be applied to other ribbon using printing apparatus.
The ribbon for thermal printing apparatus tends to be thin both to enable a large quantity of ribbon be to stored on a ribbon storage spool, as well as to ensure the efficient removal of pixels of marking medium from the ribbon. Such ribbon thinness makes the ribbon prone to breakage where ribbon tension changes to outside of tension limits, which breakage especially in an industrial context means production down time.
The ribbon tension also has a direct effect on printing quality, and the speed at which printing may be performed.
It is therefore desirable in a printing apparatus closely to control the ribbon tension.